AIN (Anterior Interosseous Nerve) Injury
What is an Anterior Interosseous Nerve (AIN) Injury?
An Anterior Interosseous Nerve (AIN) injury involves damage or dysfunction of a branch of the median nerve that supplies motor function—but not sensation—to specific muscles in the forearm. The AIN controls the flexor pollicis longus (thumb flexion), the lateral half of the flexor digitorum profundus (index finger flexion), and the pronator quadratus (forearm rotation). Injury to the AIN results in weakness or paralysis of these muscles, leading to a characteristic loss of fine motor skills, such as pinch grip.
Causes of Anterior Interosseous Nerve Injury
AIN injuries can arise from a range of traumatic and non-traumatic conditions. Common causes include:
- Compression or entrapment, such as in the deep head of the pronator teres
- Overuse or repetitive strain, particularly in athletes or manual laborers
- Direct trauma, including forearm lacerations or blunt force injuries
- Neuritis or inflammation, often seen in Parsonage-Turner syndrome (brachial neuritis)
- Post-surgical complications, especially after procedures involving the elbow or forearm
What are the Symptoms of Anterior Interosseous Nerve Injury?
AIN injury presents uniquely compared to other nerve injuries, as it is purely motor with no sensory involvement. Common symptoms include:
- Inability to pinch the thumb and index finger, often seen as a "pinch deformity" or weak OK sign
- Weakness in thumb and index finger flexion
- Difficulty with fine motor tasks, such as writing, buttoning, or holding objects
- Forearm pain, especially early in the condition, though it may subside
- No numbness or tingling, distinguishing it from other median nerve issues
These signs are important for differentiating AIN injury from carpal tunnel or other neuropathies.
How is Anterior Interosseous Nerve Injury Diagnosed?
Accurate diagnosis involves both clinical and diagnostic tools:
- Physical examination, including the "OK sign" test for pinch strength
- Strength testing, of specific forearm flexors
- Electromyography (EMG) and nerve conduction studies, to confirm motor involvement and rule out other conditions
- MRI, if a mass or structural entrapment is suspected
- Ultrasound, to detect compressive lesions or inflammation
What are the Treatment Options for Anterior Interosseous Nerve Injury?
Management depends on the severity and underlying cause:
- Rest and activity modification, particularly for overuse injuries
- Physical and occupational therapy, to maintain range of motion and retrain muscle function
- NSAIDs, for inflammation and discomfort
- Corticosteroid injections, in select compressive cases
- Surgical decompression, if there is evidence of nerve entrapment or no improvement after conservative care
Nerve transfer or graft, in rare, prolonged, or severe cases